Hydroxyapatite and related calcium phosphates have been known for many years as being suitable materials for prosthetic implants. In large part, this is due to their similarity with the mineral phase of bone. As it turns out, such materials, and especially hydroxyapatite, are widely used as bone substitutes in oral, periodontal and craniofacial surgery, and increasingly now there is a growing interest for their use in orthopaedic applications such as bone replacements, spinal fusions, joint surgery and the like.
From the point of view of biocompatibility, hydroxyapatite seems to be the most suitable ceramic material for hard tissue replacement implants. Importantly, such ceramics, along with others, do not exhibit any cytotoxic effects.
Over the years, various methods have been developed for the production of hydroxyapatite. One fairly successful approach has involved the hydrothermal conversion of corals. Indeed, it is well-known to convert coral to hydroxyapatite via hydrothermal conversion.
Corals were first converted using the hydrothermal method in 1974 by Roy and Linnehan [D M Roy (1974) “Hydroxyapatite formed from Coral Skeletal Carbonate by Hydrothermal Exchange 247 Nature, 220-222]. According to that experiment, complete replacement of aragonite by phosphatic material was achieved at a temperature of 270° C. and a pressure of 103 MPa. However, the Roy method has the disadvantage that the hydrothermal treatment must be carried out at a relatively high temperature and pressure, and the resultant hydroxyapatite may have reduced strength.
But it is not only with the Roy method that the resultant hydroxyapatite has been fragile. Clearly, where the hydroxyapatite is to be used for the development of prosthetic implants, it is undesirable to have excessive fragility.
In 1990, U.S. Pat. No. 4,938,938 disclosed a new method for the hydrothermal conversion process which included a series of cleansing steps to remove impurities from the coral prior to conversion. The disclosure of that specification suggests that the inclusion of such cleansing steps allows for a process with reduced energy requirements and relatively low pressure, which can be carried out over a relatively short period of time, and is nevertheless capable of providing a resultant hydroxyapatite with a desirable bone-like defect density and cryptocrystallinity.
Later the same year, U.S. Pat. No. 4,976,736 also disclosed a process for the production of hydroxyapatite which could be performed at lower temperatures and pressures than that of the process described by Roy. The citation provides a detailed analysis of the way hydroxyapatite formed according to the invention can be used during oral, periodontal, craniofacial and orthopaedic surgery. The real thrust of the invention, is the creation of a material having a base portion of calcium carbonate and a surface layer of a synthetic phosphate, including in some embodiments, hydroxyapatite. Apparently, where bone substitutes are produced according to this invention, it is essentially possible to ‘pre-programme’ the rate at which the ceramic implant degrades, and, therefore, possible to provide surgeons with a choice as to the rate at which the bone substitute is resorbed following implantation.
More recently in 1996, Sivakumar et al. (17 Biomaterials 1709) developed a new method for the hydrothermal conversion of Indian corals.
Owing to the unfavourable mechanical properties of porous hydroxyapatite (HAp), almost all research to date has been focused on the development of calcium phosphate coatings and particulate reinforced composites. Sol gel technology offers an improved alternative technique for producing bioactive surfaces for better bone attachment. Previously published works indicate that although biphasic hydroxyapatite products are available, monophasic hydroxyapatite powders and coatings were difficult to produce.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.